Alkali metal halides, such as lithium bromide, are used in absorbent fluids in absorption refrigeration machines. An exemplary alkali metal halide solution for absorption refrigeration applications is a lithium bromide solution, adjusted to pH range of 7-13 with lithium hydroxide.
Although this and other types of absorption solutions can be advantageous for the refrigeration cycles, alkali metal halides can be corrosive toward the materials used to construct the refrigeration machine. Such materials can include mild and stainless steel for containment components and copper or copper-nickel alloys for tube bundles, among others.
In addition to the surface damage caused by corrosion, the corrosion reaction evolves hydrogen gas as a byproduct. Incondensibles in the form of atoms or ions can easily enter and diffuse into metals, resulting in the degradation of their mechanical properties under certain conditions.
The severity of corrosion can vary depending upon factors such as temperatures of the system, concentration of alkali metal halide in the absorption solution, metals used in the construction of the refrigeration unit, the presence of air, and the like. For example, during use, the internal temperature of such machines can be high, typically up to about 450.degree. F. and higher, depending on the type of the absorption cycle, which can increase the corrosive effect of the alkali metal halide solution.
Various additives, such as lithium chromate, lithium nitrate, and lithium molybdate, have been proposed as corrosion inhibitors in alkali metal halide absorption solutions. However, lithium chromate can raise environmental concerns, and its use is being phased out. Further, the level of chromate and its oxidation state must be carefully maintained. If too little chromate is used, then it does not properly passivate the whole metal surface and pitting can result. Lithium nitrate can potentially evolve ammonia, which can cause stress corrosion cracking of copper based alloys such as heat exchanger tubes. Lithium molybdate exhibits only limited solubility in alkali metal halide solutions. In addition, lithium molybdate is metastable in aqueous halide solutions. Thus it can be difficult to maintain in aqueous halide solutions and to maintain a constant concentration of molybdate ions in solution.
Pending U.S. patent application Ser. No. 08/882,771, filed Jun. 26, 1997, titled "Corrosion Inhibiting Solutions for Refrigeration Systems," is directed to the use of heteropoly complex anions of transitional elements as corrosion inhibition additives. Refrigeration absorption fluid formulations which included heteropoly complex anions as a component exhibited improved corrosion inhibition properties and low generation of hydrogen gas that is evolved due to the corrosion reaction of alkali halide solutions with the materials of construction of the refrigeration machine. Pending U.S. patent application Ser. No. 08/896,110, filed Jul. 17, 1997, titled "Corrosion Inhibiting Solutions for Refrigeration Systems," is directed to the use of halide compounds of metallic elements of group Va, and demonstrated that alkali halide formulations containing these elements offer improved corrosion protection and low hydrogen as compared to conventional corrosion inhibitors.